<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    

    <input type="text" placeholder="半径" id="a">
    <input type="text" placeholder="周长" id="b">
    <input type="text" placeholder="面积" id="c">

    <script>
      
        //根据圆的半径
        var p = Math.PI;
        console.log(p);

        document.querySelector("#a").value = 1;
        document.querySelector("#b").value = 2;
        document.querySelector("#c").value = 3;

    
    </script>

</body>
</html>